In a typical combined cycle power plant, hot exhaust gas from a gas turbine is introduced to a heat recovery steam generator (HRSG), to convert water to steam for admission into a steam turbine. The steam turbine includes a high pressure (HP) turbine section, an intermediate pressure (IP) turbine section and a low pressure (LP) turbine section. Steam is admitted into these turbine sections through admission pipes via one or more admission valves at the turbine sections. A minimum operating pressure of the HRSG, known as a floor pressure, is determined based on admission pipe size and desired HRSG cost considerations. During operation of the system, it is desired to maintain a pressure in the LP turbine section equal to or below LP admission pipe pressure. LP admission floor pressure in the HRSG is typically about 5% below LP pressure during full power operation. During full power operation, this is not problematic since the LP floor pressure is lower than pressure in the LP section. During part-power operations of the steam turbine, for example, 40% power, the LP section pressure lowers proportionally from the LP section pressure at full load. The HRSG, however, is still providing pressure to the LP admission valve at the LP floor pressure, which is greater than the part-power LP section pressure. In such situations, the LP admission valve must throttle the flow through the LP admission valve and LP admission pipe to be equal to or higher than the LP pressure. This throttling of the LP admission valve represents a performance loss to the HRSG and steam turbine system.
In a typical combined cycle power plant, hot exhaust gas from a gas turbine is introduced to a heat recovery steam generator (HRSG), to convert water to steam for admission into a steam turbine. The steam turbine includes a high pressure (HP) turbine section, an intermediate pressure (IP) turbine section and a low pressure (LP) turbine section. Steam is admitted into these turbine sections through admission pipes via one or more admission valves at the turbine sections. A minimum operating pressure of the HRSG, known as a floor pressure, is determined based on admission pipe size and desired HRSG cost considerations. During operation of the system, it is desired to maintain a pressure in the LP turbine section equal to or below LP admission pipe pressure. LP admission floor pressure in the HRSG is typically about 5% below LP pressure during full power operation. During full power operation, this is not problematic since the LP floor pressure is lower than pressure in the LP section. During part-power operations of the steam turbine, for example, 40% power, the LP section pressure lowers proportionally from the LP section pressure at full load. The HRSG, however, is still providing pressure to the LP admission valve at the LP floor pressure, which is greater than the part-power LP section pressure. In such situations, the LP admission valve must throttle the flow through the LP admission valve and LP admission pipe to be equal to or higher than the LP pressure. This throttling of the LP admission valve represents a performance loss to the HRSG and steam turbine system.